The present invention relates to a roll for a paper, board or a finishing machine. More particularly, the invention relates to a composite roll supported at the middle thereof. Most preferably, the composite roll of the invention is used as a spreader roll or a calender fly roll in a paper or board machine. The invention also relates to a new type of supporting the roll and a drive of the roll in a hypercritical speed range.
Rolls as mentioned above, which are supported at the middle thereof are primarily used as lead rolls, spreader rolls and fly rolls in the paper industry. They feature deflection properties which are different from those of conventional rolls supported at the ends thereof. Hence, for example length differences between the edge and the middle section of the fabric can be compensated with them, and it can be assured that the fabric or the web will not be wrinkled, by bending the roll into a bow form, away from the direction of approach, spreader roll.
A type of spreader roll is disclosed in FI patent publication 72766 in which the roll is formed by two concentric cylindrical shells being spaced from each other when in a state of rest; sometimes they are also called inner and outer shells, which are interconnected at the middle of the roll in the length direction thereof. The inner shell is provided with shaft journals which are rotatably mounted on bearings. When a roll of the above-described type is rotated in a paper machine or equivalent, either the machine fabric or the web, or both of them together, make a deflection of some degree to the inner shell of the roll. The deflection is, however, not transferred to the outer shell, which is attached to the inner shell only around the middle of the roll, as explained above, but it tends to maintain its straight cylindrical shape. Depending merely on the stiffness of the outer shell, it can either stay straight in operation or bend in the opposite direction in view of the inner shell. In both cases, the risk of the fabric or the web becoming wrinkled has been avoided.
It is also noteworthy, with regard to said FI patent publication 72766, that it relates to a composite spreader roll, which is made of, e.g. carbon fiber reinforced epoxy resin.
Calender fly rolls are discussed, e.g., in U.S. Pat. Nos. 4,692,971 and 5,438,920; and patent applications WO-A1-9909329 and WO-A1-9742375. Their structure greatly resembles that of the above-described spreader roll, at least in so far as their structure relates to the present invention.
Further, an arrangement suitable for operating as both a spreader roll and a fly roll is discussed in the Metso Paper, Inc.'s FI patent application 20031384, which describes a roll, the inner shell whereof is conventionally mounted on bearings at the frame structure of the machine. The actual invention of the application is the supporting of the outer shell at the ends thereof through an adjustable bearing arrangement on the frame structure of the machine in such a manner that both the direction and the amount of the deflection of the outer shell are controllable.
The rolls supported at the middle thereof as described hereinabove feature relatively flexible outer shells and relatively stiff inner shells. This property has been brought about through inter-dimensioning of the inner and outer shells and/or through selections of the materials.
A problem often faced, however, when using these rolls, especially calender fly rolls, but also at some points where spreader rolls are used, is the critical nominal frequency of the roll. When the outer shell of a composite roll is supported by the inner shell, only, and in most cases at the middle of the roll, only, a first critical nominal frequency of the roll remains relatively low, provided that the roll diameter is maintained equal to that of the existing fly rolls or spreader rolls. In practice, this means that in order to enable use of the roll at a certain rotating speed, it should be ascertained that the rotating speed is not within the critical nominal frequency of the roll. In most cases, the rotating speed of the roll is maintained lower than the critical nominal frequency. In practice, at certain points of use where it seems that the rotating speed of the roll will coincide its critical nominal frequency, the only way to avoid it is to increase the roll diameter. However, this is not normally possible because there is no room for rolls with dimensions of that order, for example, in supercalenders.
In other words, a dimensional limitation is the most prominent problem because, for example, the calender elevator and the roll clearance have to meet certain safety regulations. A larger diameter of the roll naturally also changes the nip outlet and inlet angles.
Another problem, which is noteworthy, is that tension measuring cannot be effected at least with certain prior art spreader rolls because with these rolls, bowing of the roll directs a moment to the attachment of the roll ends, which moment significantly hampers tension measuring. Rolls of this type are formed of one cylinder, which is in most cases manufactured of composite material and which is supported at both ends thereof, at two points axially spaced from each other, on the frame structure of the paper or board machine. One of the points can be regarded as a stationary point of support, whereas through the other point the roll is subjected to a radial force, which is either pushing or pulling and causes the roll to bow. Said force is thereby also directed to the devices by which the roll is fastened to/supported by said frame structure. Besides the tension also the force/moment bowing the roll is naturally sensed by the sensor used for tension measuring, and the data from the sensor is no more valid. In other words, when this kind of a roll is used, tension has to be measured at a separate roll.
Thus, an object of the present invention is to eliminate at least part of the problems of the prior art described above.